1. Field of the Invention
The present invention relates to a method for growing thin films on substrates placed in a reaction space and subjected to alternately repeated surface reactions of a least two vapor phase reactants.
In the present method, the substrate is typically located in a reaction space, wherein it in accordance with the Atomic Layer Epitaxy (ALE) method is subjected to alternately repeated surface reactions of at least two different reactants. According to the present method, the reactants are admitted repetitively and alternately each reactant at a time from its own source in the form of vapor-phase pulses into the reaction space. Here, the vapor-phase reactants are allowed to react with the substrate surface for the purpose of forming a solid-state thin film on the substrate.
While the method is most appropriately suited for producing so-called compound thin films using as the reactants such starting materials that contain component elements of the desired compound thin-film, it may also be applied to growing elemental thin films. Of compound films typically used in the art, reference can be made to ZnS films employed in electroluminescent displays, whereby such films are grown on a glass substrate using zinc sulfide and hydrogen sulfide as the reactants in the growth process. Of elemental thin films, reference can be made to silicon thin films.
The invention also concerns an apparatus comprising a reaction chamber with gas flow channels suited for an inflow of vapor phase reactant pulses and an outflow of reaction products, wherein at least a portion of the gas flow channels have a narrow, oblong cross section for minimizing the volume of the reaction space suited for producing thin films.
The apparatus comprises a reaction space into which the substrate can be placed, and at least two reactant sources from which the reactants used in the thin-film growth process can be fed in the form of vapor-phase pulses into the reaction space. The sources are connected to the reaction space via reactant inflow channels, and outflow channels are connected to the reaction space for removing
2. Related Art
The gaseous reaction products of the thin-film growth process as well as the excess reactants in vapor phase.
Conventionally, thin-films are grown using vacuum evaporation deposition, the Molecular Beam Epitaxy (MBE) and other vacuum deposition methods, different variants of the Chemical Vapor Dcposition (CVD) method, including low-pressure and metal-organic CVD and plasma-enhanced CVD, or alternatively, the above-described deposition method of alternately repeated surface reactions called the Atomic Layer Epitaxy (ALE) method. In the MBE and CVD methods, besides other process variables, the thin-film growth rate is also affected by the concentrations of the starting material inflows. To achieve a uniform thickness of the layers deposited by the first category of conventional methods, the concentrations and reactivities of starting materials must hence be carefully kept constant all over the substrate area. If the starting materials are allowed to mix with each other prior to reaching the substrate surface as is the case in the CVD method, for instance, a chance of their premature mutual reaction arises. Then, the risk of microparticle formation already within the inflow channels of the gaseous reactants is imminent. Such microparticles have a deteriorating effect on the quality of the thin film growth. Therefore, the possibility of premature reactions in MBE and CVD reactors is avoided by heating the starting materials no earlier than at the substrate surfaces. In addition to heating, the desired reaction can be initiated using, e.g., a plasma or other similar activating means.
In the MBE and CVD processes, the growth of thin films is primarily adjusted by controlling the inflow rates of starting materials impinging on the substrate. By contrast, the ALE process is based on allowing the substrate surface qualities rather than the starting material concentrations or flow variables to control the deposition rate. The only prerequisite in the ALE process is that the starting material is available in sufficient concentration for thin-film formation on all sides of the substrate.
The ALE method is described in the FI patent publications 52,359 and 57,975 and in the U.S. Pat. Nos. 4,058,430 and 4,389,973, in which also some apparatus embodiments suited to implement this method are disclosed. Apparatus constructions for growing thin films are also to be found in the following publications: Material Science Reports 4(7) (1989), p. 261, and Tyhjiotekniikka (Finnish publication for vacuum techniques), ISBN 951-794-422-5, pp. 253-261.
In the ALE growth method, atoms or molecules are arranged to sweep over the substrates thus continuously impinging on their surface so that a fully saturated molecular layer is formed thereon. According to the conventional techniques known from the FI patent publication No. 57,975, the saturation step is followed by an inert gas pulse forming a diffusion barrier which sweeps away the excess starting material and the gaseous reaction products from above the substrate. The successive pulses of different starting materials and of diffusion barriers of an inert gas separating the former accomplish the growth of the thin film at a rate controlled by the surface chemistry properties of the different materials. Such a reactor is called a "traveling-wave" reactor. For the function of the process it is irrelevant whether the gases or the substrates are moved, but rather, it is imperative that the different starting materials of the successive reaction steps are separated from each other and arranged to impinge on the substrate successively.
Most vacuum evaporators operate on the so-called "single-shot" principle. Herein, a vaporized atom or molecule species can impinge on the substrate only once. If no reaction of the species with the substrate surface occurs, the species is bounced or re-vaporized so as to hit the apparatus walls or the inlet to the vacuum pump undergoing condensation therein. In hot-wall reactors, an atom or molecule species impinging on the reactor wall or the substrate may become re-vaporized, whereby advantageous conditions are created for repeated impingements of the species on the substrate. When applied to ALE reactors, this "multi-shot" principle can provide, i.a., improved material utilization efficiency.
In conventional ALE apparatuses, a characterizing property is that the different starting materials of the reaction are understood to be isolated from each other by means of a diffusion wall formed by an inert gas zone traveling between two successive pulses of starting materials, cf. above-cited FI patent publication No. 57,975 and the corresponding U.S. Pat. No. 4,389,973. The length of the inert gas zone acting as the downstream flowing diffusion wall is such that only approx. one millionth of the reactant gas molecules have a sufficient diffusion velocity to travel under the prevailing conditions in the counterflow direction to a distance greater than the thickness of the isolating diffusion wall employed in the method.
However, notwithstanding the high reliability of the above-described arrangement, it has some disadvantages. For instance, the cross sections and shapes of piping in practical reactor constructions vary between, e.g., the infeed manifold and the substrates, whereby the thickness and shape of the diffusion wall become difficult to control and the starting materials may become carried over into contact with each other. The diffusion wall may also become destroyed in the nozzles feeding the vapor-phase reactant to the substrates, in gas mixers or at other discontinuity points of the piping. The laminarity of gas inflow may also become disturbed by a too tight bend in the piping.
Intermixing of starting materials in flow systems cannot be prevented simply by keeping the gas volumes apart from each other, because mixing may also occur due to adherence of molecules from a starting material pulse on the apparatus walls or discontinuities thereof, wherefrom the molecules may then gain access with the molecules of the successive starting material pulse.
It is an object of the present invention to overcome the drawbacks of conventional technology and to provide an entirely novel arrangement for growing thin films.